2011 mitsubishi lancer fuse voice system not working fully and how do i get it to work right?

So heres the deal. Previous owner of the car istalled an amp under the drivers seat and all the speaker wires are going to it. Sound in the car is amazing. Whats not so amazing is whatever they did while hooking this amp to the head unit they messed up the fuse voice system. It still works but is unusable because the volume is so much lower than it is supposed to be. Any suggestions on how this happened or how to fix the problem?